Fluid flow meters are used in industrial process control environments to measure fluid flow and provide flow signals for flow indicators and controllers. Inferential flow meters measure fluid flow in a pipe by measuring a pressure drop near a discontinuity within the pipe. The discontinuity (primary element) can be an orifice, a nozzle, a venturi, a pitot tube, a vortex shedding bar, a target or even a simple bend in the pipe. Flow around the discontinuity causes both a pressure drop and increased turbulence. The pressure drop is sensed by a pressure transmitter (secondary element) placed outside the pipe and connected by impulse lines or impulse passageways to the fluid in the pipe. Reliability depends on maintaining a correct calibration. Erosion or buildup of solids on the primary element can change the calibration. Impulse lines can become plugged over time, which also adversely affects calibration.
Disassembly and inspection of the impulse lines is one method used to detect and correct plugging of lines. Another known method for detecting plugging is to periodically add a xe2x80x9ccheck pulsexe2x80x9d to the measurement signal from a pressure transmitter. This check pulse causes a control system connected to the transmitter to disturb the flow. If the pressure transmitter fails to accurately sense the flow disturbance, an alarm signal is generated indicating line plugging. Another known method for detecting plugging is sensing of both static and differential pressures. If there is inadequate correlation between oscillations in the static and differential pressures, then an alarm signal is generated indicating line plugging. Still another known method for detecting line plugging is to sense static pressures and pass them through high pass and low pass filters. Noise signals obtained from the filters are compared to a threshold, and if variance in the noise is less than the threshold, then an alarm signal indicates that the line is blocked.
These known methods rely on providing static pressure sensors or disassembly of the flow meter or use of an external control system for diagnostics, increasing complexity and reducing reliability. These known methods do not provide for diagnosing the condition of the primary element. There is thus a need for a better diagnostic technology providing more predictive, less reactive maintenance for reducing cost or improving reliability.
A fluid flow meter diagnoses the condition of its primary element or impulse lines. The primary element and the impulse lines together form a differential pressure generator. This differential pressure generator generates a differential pressure that represents the flow rate. The differential pressure is coupled to a differential pressure sensor in the fluid flow meter.
A difference circuit coupled to the differential pressure sensor generates a difference output representing the sensed differential pressure minus a moving average of the sensed differential pressure.
A calculate circuit receives the difference output and calculates a trained output of historical data obtained during an initial training time. The calculate circuit also calculates a monitor output of current data obtained during monitoring or normal operation of the fluid flow meter.
A diagnostic circuit receives the trained output and the monitor output and generates a diagnostic output indicating a current condition of the pressure generator relative to an historical condition.
A flow circuit is also coupled to the sensor and generates an output indicating the flow rate.